There is a requirement for future gas turbine engines to have increased performance, thermodynamic efficiency and component cyclic life, maintained component integrity and reduced weight and cost. This requires increased pressure ratio in the compressor, increased turbine entry temperature and increased turbine speed. The increase in pressure ratio in the compressor requires the compressor rotor disc to operate at higher temperatures. The increase in turbine entry temperature requires the turbine rotor disc to operate at higher temperature. The increase in turbine speed requires the turbine rotor disc to operate at higher stresses. The above requirements result in the need for high pressure compressor rotor discs and turbine rotor discs capable of operating at increased temperature and having increased strength.
Nickel base superalloys of high strength, around 1500 Mpa, and increased temperature capability, above 700° C., must maintain damage tolerance. As a result of normal operation, rotor discs are subject to cyclic mechanical stresses and contain features, such as bolt holes, which represent a stress concentration and are potential sites for fatigue damage. The rotor discs are also exposed to thermal gradients leading to exposure to thermal stress patterns. The greatest temperature is at the rim of the rotor disc. The rotor discs therefore must maintain a high level of creep resistance to prevent distortion in addition to resistance to fatigue.
The operating requirements placed on the rotor disc depend on two factors. Firstly, whether the rotor disc is a turbine rotor disc or a high pressure compressor rotor disc. Secondly, whether the gas turbine engine is an aero gas turbine engine, a marine gas turbine engine or an industrial gas turbine engine. The rotor discs of an industrial gas turbine engine require a relatively low cycle life compared to the rotor discs of an aero gas turbine engine. The rotor discs of an industrial gas turbine engine are more susceptible to creep damage and microstructural degradation compared to the rotor discs of an aero gas turbine engine. This difference arises because an industrial gas turbine engine operates for 100's of 1000's of hours compared to 10's of 1000's of hours for an aero gas turbine engine.
Gas turbine engine rotor discs are currently manufactured from nickel base superalloys such as Waspaloy, Udimet 720Li and RR1000. Waspaloy has high fatigue crack propagation resistance, phase stability, processing ability and is of relatively low cost. However Waspaloy has relatively low strength. The relative strength of Waspaloy is directly related to the gamma prime fraction of Waspaloy, which contains 24% volume fraction gamma prime phase. Udimet 720Li has fatigue crack propagation resistance less than Waspaloy, but has higher strength than Waspaloy. The high, 45 wt %, gamma prime phase fraction in Udimet 720Li is responsible for the higher strength. RR1000 has fatigue crack propagation resistance similar to Waspaloy, but has creep and tensile strength higher than Waspaloy. The high, 48 wt %, gamma prime phase fraction in RR1000 is responsible for the higher strength. RR1000 has similar strength to Udimet 720Li, but has greater fatigue crack propagation resistance and creep rupture life. However, RR1000 is relatively expensive compared to Waspaloy and Udimet 720Li due to its highly alloyed composition. Waspaloy and Udimet 720Li can be manufactured by powder metallurgy processing or by cast and wrought processing. RR1000 is currently manufactured by powder metallurgy processing which minimises segregation and has improved ultrasonic inspectability compared to the cast and wrought route.